1. Field of the Invention
The present invention relates to data retrieval systems, and more particularly to an apparatus and method for detecting a maximum mark length recorded on an optical disk.
2. Description of Related Art
In an optical disk such as a compact disc (CD) or a digital versatile disc (DVD), the frequency of signals read from the disk vary due to disk rotation speed variations. Also, the greater the radial distance of a track to the center of the disk the longer its length. Accordingly, in a constant angular velocity (CAV) system, the linear velocity is greater toward the outer circumference of the disk. As a result, even when the disk is rotated at a constant speed, a signal read from the outer circumference has a higher frequency. In general, the frequency may vary over an operating range of approximately xc2x130% to xc2x150%.
As a general countermeasure against such variation in the frequency, a reference or basic frequency is calculated based on signals recorded on the disk, and then the calculated reference frequency is set as the center frequency of a phase-locked loop (PLL). The reference frequency is generally calculated by using a maximum mark length obtained in a frame synchronous area (sync area) which are placed at regular intervals in the frames. A sync area as used herein means an area where the starting position of a data area is defined by a code not existing in the data area. In such a sync area, a frame number (frame ID) and a frame sync code used for frame synchronization are written. The frame sync code includes a longest mark for the synchronization (frame synchronization) that is longer than any other mark in the data areas. This longest mark is usually detected for computing the reference frequency.
In an optical disk such as a DVD, data are spirally or concentrically recorded on the surface of the disk. The data are recorded by using marks each having a length along the circumferential (tracking) direction. A mark is detected by using a difference in the quantity of reflected light depending upon the presence or absence of a mark (that is, a pit or a land) under irradiation with a laser beam or the like. For example, a mark can be detected on the basis of an interval between crossing points of a preset slice level and the quantity of reflected light (output waveform). In general, the length of a mark (hereinafter referred to as the xe2x80x9cmark lengthxe2x80x9d) is represented by using a time interval nT between the crossing points, where T indicates time per bit (namely, a fundamental period), and n is an integer. The mark is recorded with a length of an integral multiple of the fundamental period T. The reference frequency can be obtained as 1/T based on the fundamental period T. Hereinafter, a mark with a mark length of nT is expressed as a mark nT.
A conventional maximum mark length detector has, for example, a structure as shown in FIG. 6. The maximum mark length detector 90 comprises a measuring device 92 for measuring mark lengths Lk, a maximum value register 94 for storing a maximum value Lmax of measured mark lengths Lk, and a comparator 96 for comparing the maximum value Lmax stored in the maximum value register 94 with a measured mark length Lk. When the measured mark length Lk is greater than the current maximum value Lmax in the register 94, a control unit 98 substitutes the measured mark length Lk for the maximum value Lmax in the maximum value register 94.
The maximum mark length can be detected, for example, by using a procedure as shown in FIG. 7 in which the detection is conducted twice. It is assumed that a first maximum mark length Lmax1 is first detected (step S180), and subsequently, a second maximum mark length Lmax2 is detected (step S182). Then, the comparator 96 compares these maximum mark lengths Lmax1 and Lmax2 (step S184). When they are equal to each other, the detection is completed because the maximum mark length has been detected. In contrast, when they are different from each other, the detection is started over again. The measurement is carried out twice in order to confirm the value of the maximum mark length Lmax1. On the basis of the maximum mark length thus detected, the reference frequency of signals to be read from the disk can be obtained. However, usually the maximum mark lengths Lmax1 and Lmax2 do not exactly accord with each other due to the influence of measurement error, noise and the like. Therefore, typically they are considered to be equivalent, when:
Lmax1xe2x88x92Lmax2 less than xcex94L
where xcex94L indicates a tolerance which takes into account measurement error, noise and the like.
In the detection of the maximum mark lengths (step S180 and step S182), a mark detection time Tw is set, which has a duration of an integral multiple of a system clock and is selected to necessarily include a maximum length mark so that a maximum mark length can be detected within the detection time Tw. The detection (step S180 and step S182) can be carried out by using a procedure as shown in FIG. 8, for example. First, when measuring device 92 provides a new measured mark length (measured value Lk) (step S192), the measured value Lk is compared with a current maximum value Lmax in the register 94 (step S194). When the measured value Lk is greater than the current maximum value Lmax, the measured value Lk is substituted for the maximum value Lmax in the register 94 as a new current maximum value (step S196). The maximum value Lmax in the register 94 is initially set to zero (step S190). Subsequently, the comparator 96 compares the elapsed time count measured by control unit 98 with the mark detection time Tw (step S198). When the measured elapsed time reaches the detection time Tw, the measurement of mark lengths is completed. In contrast, when the measured elapsed time does not reach the detection time Tw, a subsequent mark length is measured (step S192).
As described above, in the prior art, a fixed value related to the system clock has been used as the mark detection time Tw. However, as is shown in FIG. 9, when a disk 60 is rotated at a constant speed, the time interval between two sync areas 62 (hereinafter referred to as the xe2x80x9csync area intervalxe2x80x9d) varies due to the difference in the linear velocity between the inner circumference and the outer circumference. Accordingly, a signal read from the outer circumference has a higher frequency.
FIG. 9, a reference numeral 64 denotes a data area, Tso indicates a sync area interval in the outermost circumference and Tsi indicates a sync area interval in the innermost circumference. Furthermore, the mark detection time Tw is required to include at least one sync area 62 (where a longest mark is recorded). Therefore, the mark detection time Tw is set on the basis of the sync area interval in the innermost circumference where the lowest frequency is obtained.
When such a fixed mark detection time Tw is used, as shown in FIG. 9, for example, the mark detection time Tw includes merely one sync area 62 in the innermost circumference but includes plural (seven in FIG. 9) sync areas 62 in the outermost circumference. However, one sync area 62 is sufficient for the detection. Therefore, in the outermost circumference, the maximum mark length detection is carried out even in the superfluous six sync areas 62. Accordingly, six sevenths of the detection time is wasted in the outermost circumference. This increases the wait time before the start of a data read operation.
Moreover, a mark length is detected on the basis of the interval between the crossing points of the output waveform obtained from the disk and the slice level. Therefore, referring to an output waveform 70 shown in FIG. 10, when the slice level lowers (as is shown with a reference numeral 74), two consecutive mark lengths Lxe2x80x2 (k-1) and Lxe2x80x2 (k) to be measured are:
Lxe2x80x2(k xe2x88x921)=L(kxe2x88x921)xe2x88x922xcex94T
Lxe2x80x2(k)=L(k)+2xcex94T
where L(kxe2x88x921) and L(k) indicate measured values obtained when the slice level is correct (as shown with a reference numeral 72), and xcex94T indicates a measurement error of the mark lengths derived from an error of the slice level. In this manner, the variation of the slice level leads to an error in the measurement of a mark length. Therefore, even when marks with the same mark length are compared, there is a possibility that these marks are identified as different marks due to the error in the measured mark lengths. Furthermore, if the measured maximum mark length includes an error, then that error causes errors in the calculated reference frequency.
Accordingly, it is an object of the present invention to solve the aforementioned problems.
The present invention provides a system of detecting a maximum mark length used for computing a reference frequency for signals to be read from a disk medium in which data are recorded by using predetermined marks having different lengths in a circumferential direction. Sync areas spaced at a predetermined interval in the circumferential direction each include a mark pair comprising a longest mark and a short mark subsequent to the longest mark. A maximum mark length is detected by: storing a measured mark length; measuring the length of a mark subsequent to the measured mark; computing the sum, referred to as a mark pair, of the stored mark length and the subsequently measured mark length; computing a ratio between the sum and a current measured mark length; and comparing the ratio with a predetermined value to determine whether the sum corresponds to a maximum mark pair length. The maximum detected mark pair length in a sync area is then used for computing the reference frequency.
A maximum mark length detector detects a maximum mark length used for computing a reference frequency for signals to be read from a disk medium in which data are recorded by using predetermined marks respectively having different lengths in a circumferential direction and sync areas spaced at a predetermined interval in the circumferential direction. Each sync area includes a mark pair comprising a longest mark and a short mark subsequent to said longest mark.
The maximum mark length detector comprises a measured value register for storing a measured mark length and an adder for computing the sum of a current measured mark length and a previously measured mark length stored in the measured value register, wherein the length of the mark pair given by the sum of the length of the longest mark and the length of the short mark is detected as a maximum mark pair length and used as the maximum mark length for computing the reference frequency.
Moreover, the method of detecting a maximum mark length according to the present invention further comprises the steps of determining the total value of mark lengths measured after a current maximum value of the sum of the measured mark lengths is newly detected, and the step of completing detection when the total value reaches a value corresponding to an interval between the longest marks.
The maximum mark length detector further comprises a total value register for storing a total value of mark lengths measured after a new current maximum value of the sum of the measured mark lengths is detected, an adder for adding a measured value to the total value, and a detection completion logic unit to determine the method of detecting a maximum mark length is complete when the total value reaches a value corresponding to an interval between two of the longest marks.